1. Field of the Invention
The present invention relates to a lens barrel and an imaging apparatus, and particularly to a lens barrel employing what is called a collapsible structure.
2. Description of the Related Art
As recording means in a digital still camera, there has been a known method in which an imaging device formed of photoelectric conversion elements, such as a CCD (Charge Coupled Device) sensor and a CMOS (Complementary Metal Oxide Semiconductor) sensor, converts the amount of light of a subject image formed on a surface of the imaging device into an electric output, which is then recorded.
As micro-processing technologies have been progressing in recent years, CPUs (Central Processing Units) have run faster and storage media have increased their packing densities, whereby a large amount of image data that has not been handled before can now be processed at high speed.
Light receiving devices have also increased their packing densities and shrunk in size. The higher packing densities allow higher spatial frequency recording, and the size reduction of the light receiving devices allows cameras to be smaller as a whole.
The higher packing densities and size reduction described above, however, disadvantageously reduce the light receiving area of each photoelectric conversion element but increase its susceptibility to noise as its electric output decreases. To address the problems, the amount of light that reaches each of the photoelectric conversion elements is increased by increasing the aperture diameter of an optical system, and tiny lens elements (what is called a microlens array) are disposed in front of the respective photoelectric conversion elements.
A light flux directed to the portion between adjacent photoelectric conversion elements when there is no microlens array is guided by a microlens array to these photoelectric conversion elements, but the micro lens array constrains the exit pupil position of a lens system (distance from the image plane to the exit pupil).
The reason for this is that when the exit pupil position of the lens system approaches the photoelectric conversion elements, that is, when the principal ray that reaches each of the photoelectric conversion elements forms a large angle with the optical axis of the lens system, off-axis light fluxes directed toward the periphery of the screen are inclined to the optical axis by large angles and hence do not reach peripheral photoelectric conversion elements, resulting in an insufficient amount of light.
In recent years, as digital still cameras have been widely used, customer needs have become diverse. In particular, small digital still cameras equipped with zoom lenses having high variable power ratios have become popular, and so have zoom lenses having a variable power ratio as high as 5 or greater.
A zoom lens type is often categorized by a zoom lens type representing the configuration of the zoom lens, and a zoom lens having a high variable power ratio has typically employed a zoom lens type formed of four lens groups, positive, negative, positive, and positive lens groups. The zoom lens type formed of four lens groups, positive, negative, positive, and positive lens groups, includes a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power, and a fourth lens group having positive refracting power arranged in this order from the object side.
In a zoom lens employing the zoom lens type formed of four lens groups, positive, negative, positive, and positive lens groups, when the lens position setting representing the positions of the lenses that form the zoom lens is changed from a wide angle end state in which the focal length of the zoom lens has the smallest value to a telescopic end state in which the focal length has the greatest value, the first to third lens groups are moved in such a way that the distance between the first and second lens groups increases and the distance between the second and third lens groups decreases, and the fourth lens group is separately driven and moved in such a way that change in image plane position (focus position), which should be on the imaging device, is compensated (see JP-A-2008-146016, for example).
As a collapsible lens barrel in which at least four lens groups are driven in the optical axis direction, there has been a known lens barrel structure in which four movable lens groups can be driven, specifically, three lens groups are driven by using cams and the remaining lens group disposed in a position closest to the image plane is separately driven (see JP-A-2008-46500, for example).
A collapsible lens barrel of this type is often used in what is called a collapsible digital still camera, in which the entire lens portion can retract into the body of the camera when the camera is not in operation.
On the other hand, since it has been known that when the number of movable lens groups increases, in general, the degree of freedom in selecting how the lenses are moved in a zooming operation increases, whereby the variable power ratio can be increased or the performance of the zoom lens can be enhanced.